Such a telephone set wherein the signal generating unit transmits signalling signals over a separate pair of wires which are isolated from the pair of wires used for the transmission of speech signals to a telephone exchange connected to the subscriber's line, is inter alia disclosed in the periodical Telesis FALL 1975, pp. 84-90.
The use of contemporary electronics in telephone exchanges and in telephone sets enables a rapid mutual exchange of information. Multi-frequency tone signalling can be used with great advantage because it permits a high signalling rate, up to 10 bits per second, and telephony systems are transparent for such signals.
For still higher signalling rates the information can be transmitted as data with a high bit rate of, for example, 150 bits/second. This rapid signalling, enabled by electronics, has the considerable advantage that an extensive set of services can be offered to the subscribers with a comparatively simple operation such as: reduced dialling, the fact that the subscriber's number can move with the subscriber when the subscriber moves permanently or temporarily to place where another telephone set is available, the automatic re-dialling of a subcriber whose set was busy etc.
These functions are performed by the information processing unit which, in its simplest form, comprises a read only memory but may comprise a micro-processor or a specially designed LSI-circuit in more sophisticated sets.
A serious problem is that the reliability is considerably smaller when a very large number of electronic components are introduced relative to telephone sets having simple multi-frequency tone signalling, which is exclusively used for the transmission of dialling signals. This fact has prohibited the introduction of such telephone sets in large numbers and for more extensive networks.
It is an object of the invention to obviate this drawback and to make the reliability of such a telephone set substantially equal to that of existing telephone sets or at least equal in performances.